What are the strict aliasing rules when casting *from* a char array? - c++

Question

I'm confused by the strict aliasing rules when it comes to casting a char array to other types. I know that it is permitted to cast any object to a char array, but I'm not sure what happens the other way around.
Take a look at this:
#include <type_traits>
using namespace std;
struct{
alignas (int) char buf[sizeof(int)]; //correct?
} buf1;
alignas(int) char buf2[sizeof(int)]; //incorrect?
struct{
float f; //obviously incorrect
} buf3;
typename std::aligned_storage<sizeof(int), alignof(int)>::type buf4; //obviously correct
int main()
{
reinterpret_cast<int&>(buf1) = 1;
*reinterpret_cast<int*>(buf2) = 1;
reinterpret_cast<int&>(buf3) = 1;
reinterpret_cast<int&>(buf4) = 1;
}
Compiling using g++-5.3.0 results in warnings only on the second and third line of main:
$ g++ -fsyntax-only -O3 -std=c++14 -Wall main.cpp
main.cpp: In function ‘int main()’:
main.cpp:25:30: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
*reinterpret_cast<int*>(buf2) = 1;
^
main.cpp:26:29: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
reinterpret_cast<int&>(buf3) = 1;
^
Is gcc correct in that lines 1 and 4 are correct, while lines 2 and 3 are not? I'm fairly sure line 4 is correct (that's what aligned_storage is for), but what are the rules at play here?

Answer 1

First of all, absence of warning is not a guarantee of correctness! gcc is getting better and better at spotting problematic code, but it is still not a static analyzing tool (and those are not perfect either!)
Second of all, yes, you are not allowed to access char array through a pointer to other type.